Bile acids are one of the major components of bile. In liver cells, cholesterol is formed from acetyl coA (coenzyme A) and further primary bile acids [cholic acid (CA) and chenodeoxycholic acid (CDCA)] are formed therefrom. The primary bile acid conjugated with glycine or taurine undergoes 7.alpha. dehydroxylation by enteric bacteria in the intestinal tract and thus secondary bile acids [deoxycholic acid (DCA) and lithocholic acid (LCA)] are formed. At the same time, ursodeoxycholic acid (UDCA) is also formed.
After facilitating the absorption of fat-soluble substances in the intestinal tract, the bile acids are mostly reabsorbed at the end of the ileum and returned to the liver through the portal vein, thus repeating the closed enterohepatic circulation. Bile acids pooled in the body (mainly in the gallbladder and the intestine) amount to about 3 to 5 g. A trace amount of bile acids deviate from the enterohepatic circulation and migrate into the greater blood circulation, which causes a trace bile acid level (about 2 .mu.g/ml) in the peripheral blood of a healthy person in the morning (fasting). The blood bile acid level shows a daily variation in association with meals. Namely, the morning (fasting) blood bile acid level is the lowest and an increase is observed after each meal. After showing a decrease at the bed time, the blood bile acid level returns to the morning (fasting) level.
Regarding bile acids and digestive diseases, there have been reported a number of studies on colon cancer and fecal bile acids [refer to references 1) and 2) in the attached literature list] and blood bile acids and liver diseases [refer to reference 3) in the above-mentioned list]. To study liver diseases accompanied by a trace serum bile acid level, in particular, there have been frequently employed instrumental analyses such as GLC (gas-liquid chromatography) [refer to reference 4) in the above-mentioned list] and HPLC (high performance liquid chromatography) [refer to reference 5) in the above-mentioned list]. However these methods each involve a complicated analytical procedure and thus require a high degree of knowledge and technique.
On the other hand, there has been reported a method for assaying serum bile acids with the use of an enzyme, i.e., 3.alpha.-hydroxysteroid dehydrogenase (3.alpha.-HSD) [refer to reference 6) in the list]. Although this method has been routinely carried out as a valuable liver function test, it aims at assaying the total bile acids.
As discussed above, instrumental analyses have been widely employed for measuring bile acids as an index to digestive diseases. However these methods each involves a complicated analytical procedure and thus requires a high degree of knowledge and technique. In addition, the method for assaying serum bile acids with the use of an enzyme aims at assaying the total bile acids.